docs: def make_filesystem(blk_device, fstype='ext4', timeout=10):
      """Make a new filesystem on the specified block device."""
      count = 0
      e_noent = os.errno.ENOENT
      while not os.path.exists(blk_device):
          if count >= timeout:
              log('Gave up waiting on block device %s' % blk_device,
                  level=ERROR)
              raise IOError(e_noent, os.strerror(e_noent), blk_device)
  
          log('Waiting for block device %s to appear' % blk_device,
              level=DEBUG)
          count += 1
          time.sleep(1)
      else:
          log('Formatting block device %s as filesystem %s.' %
              (blk_device, fstype), level=INFO)
          check_call(['mkfs', '-t', fstype, blk_device])